In known variators of the half-toroidal type it has been customary for the operator to change the transmitted ratio using what has become known in the art as "tangential shift". Each carriage and its associated roller has been mounted to rotate about its so-called precession axis, so changing the radii at which the roller makes contact with the input and output discs, and so changing the transmitted ratio. Each roller and its carriage has also been mounted so that it can be controlled to move bodily along the precession axis in a direction approximately tangential to the centre circle of the torus presented by the two discs. A rotation of a roller about its precession axis, resulting in a change of transmitted ratio, is thus effected by a tangential movement, the result of which is to introduce a steering angle between the roller and the disc at the point of contact so as to steer the roller to a new ratio. It is however well recognised in the art of half-toroidal CVT's that ratio change by tangential shift imposes design problems because of the large outward forces on the roller carriages which must be reacted in bearings permitting both linear and rotary motion with the minimum of friction.
Some alternatives to such tangential shift operation for CVT's of the half-toroidal type have however been proposed. One such proposal is described in U.S. Pat. No. 3,008,337, in which the carriage of each of the rollers--three in number--spanning the toroidal cavity between the input and output discs is mounted to rotate not only about a precession axis as already described, but also about a second or "tilt" axis lying parallel to the common axis of the discs. Ratio change is effected by each roller carriage rotating about its tilt axis, so introducing a steering angle and causing rotation of the carriage about the precession axis.
This geometry makes possible a CVT of the so-called "torque-controlled" type in which the rollers tend to adopt a ratio angle at which the sum of the torques transmitted by the input and output discs matches the control force applied to the means to pivot each roller about its tilt axis. That force, in U.S. Pat. No. 3,008,337, is applied hydraulically.
It is will known in the art of toroidal-race CVT's generally, and half-toroidal designs particularly, that the CVT can be at equilibrium at a particular transmitted ratio only when the axis of rotation of each roller intersects the common axis of the discs. If this condition is not fulfilled, the rollers will have "steering angle" and must therefore be in the course of changing the transmitted ratio to a new value at which intersection is restored. The geometry of U.S. Pat. No. 3,008,337, and in particular the orientation of the precession and tilt axes, is such that equilibrium can only exist when the tilt of each roller about its tilt axis is at a mid position. In other words; any transition of the ratio angle of the rollers from one equilibrium value to another must be initiated by a departure of each roller from its mid tilt position, and can only conclude when that mid tilt position is restored as the new ratio value is attained. There is thus no unique correlation between each equilibrium ratio of the transmission and a particular orientation of each roller about its tilt axis, because every equilibrium ratio value must be accompanied by the same mid-value of tilt. This mode of operation may result in disappointing characteristics of response and especially stability.